The frictional resistance of a vessel hull as it moves through the water can constitute 45% to 90% of the total resistance experienced by the vessel and may be increased by 6% up to 80% due to fouling of the hull by algae, sea grass, barnacles, and the like. As an example, an added resistance of 30% due to moderate bio-fouling of a tanker hull can increase the fuel consumption of the vessel by up to twelve tons per day. The result is added cost to operate the vessel, as well as increased emissions.
Accordingly, there are a variety of methods employed to reduce the likelihood of bio-fouling and/or to rid vessel hulls of bio-fouling through cleaning. For example, anti-fouling hull paints and coatings are often used in an effort to decrease the likelihood of bio-fouling, but such treatments do not always work reliably and can require frequent periodic applications. Such methods can be associated with significant cost as the vessel often must be dry docked for an extensive period of time while the paint and/or coating is applied. There are also environmental concerns with anti-fouling paints and coatings.
It is common, while a vessel is dockside and/or during normal unlading conditions, that the hull be periodically cleaned manually (e.g., by scuba divers) using motorized brushes. However, the cost of such a cleaning procedure is also high as this type of cleaning effort can be needed often. In some cases, manual hull cleaning procedures are commissioned every ten to twenty months, and in other cases even sooner if needed. Unfortunately, there are significant drawbacks to manual cleaning methods. For instance, as the procedure is carried out while the vessel is dockside, cleaning residue and debris (e.g., paint chips, etc.) are disposed in the shallow waters surrounding the docks. Because of this, many jurisdictions have made this practice illegal, particularly due to the toxicity of anti-fouling paint which contaminates the water.
In response, a type of robotic hull cleaners has been proposed. The “Hismar” consortium, for example, has proposed a robotic platform for hull cleaning during normal unlading conditions. The robot is magnetically attached to the hull when the vessel is stationary and is tethered to an operator control unit, a high pressure water source, a suction subsystem, and a power subsystem. Various other tethered robots have also been proposed. Despite some of their advantages over manual cleaning procedures, most prior hull cleaning robots suffer from various shortcomings. For instance, such robots are connected to a cable and powered and controlled by an on-board power supply and control system and are able to operate only on a stationary vessel. Further, inspection techniques for determining the cleanliness of the hull are absent or inefficient.